The field of this invention is the art of making offset printing plates for use in printing through the use of electrophotographic techniques.
Offset plates are normally made photographically by complex methods requiring the use of silver halide film, photoresists, and the processing of the images to achieve their incorporation onto plates that can be used in printing presses. In recent years electrostatic techniques have also been used to make such plates but their quality is so inferior to the quality of photographically made plates that not much of this type of technique is used.
The invention herein revolves around a type of electrophotographic film that is capable of being imaged with quality and gray scale as good as if not better than that achieved by photographic techniques and with resolution that is better. The film comprises a photoconductive coating that is bonded onto a conductive substrate. The photoconductive coating may comprise radio frequency sputtered cadmium sulfide that is preferably from about 200 Angstroms to about 2 microns thick. The conductive substrate may comprise a layer of indium tin oxide about 300 to 500 Angstroms thick which is bonded to a sheet of stable polyester plastic about 0.005 inch thick. One such film is described in U.S. Pat. No. 4,025,339 issued May 24, 1977.
Because of the fact that the electrophotographic film can be exposed at high speed and transferred immediately to a plate of material such as paper, aluminum, plastic and the like, the process can be carried on inside of a cartridge that takes the place of the usual photographic film cartridge that is used with graphic arts cameras. This is convenient and eliminates considerable processing since the resulting plate made by the device is suitable for use in an offset printing press. If the plate material is plain paper, no further processing is needed since plain paper is of a hydrophyllic nature. If the plate material is other than plain paper, additional processing may be required to make the untoned portions hydrophyllic.